Hemolysins are members of a family of protein toxins that are produced by a wide variety of organisms. Some hemolysins, for example alpha hemolysins, can disrupt the integrity of a cell membrane (e.g., a host cell membrane) by forming a pore or channel in the membrane. Pores or channels that are formed in a membrane by pore forming proteins can be used to transport certain polymers (e.g., polypeptides or polynucleotides) from one side of a membrane to the other.
Alpha-hemolysin (α-HL, a-HL or alpha-HL) is a self-assembling toxin which forms an aqueous channel in the membrane of a host cell. Alpha-HL has become a principal component for the nanopore sequencing community. It has many advantageous properties including high stability, self assembly and a pore diameter which is wide enough to accommodate single stranded DNA but not double stranded DNA (Kasianowicz et al., 1996).
Previous work on DNA detection in the a-HL pore has focused on analyzing the ionic current signature as DNA translocates through the pore (Kasianowicz et al., 1996, Akeson et al., 1999, Meller et al., 2001), a very difficult task given the translocation rate (˜1 nt/μs at 100 mV) and the inherent noise in the ionic current signal. Higher specificity has been achieved in nanopore-based sensors by incorporation of probe molecules permanently tethered to the interior of the pore (Howorka et al., 2001a and Howorka et al., 2001b; Movileanu et al., 2000).
The wild-type α-HL results in significant number of deletion errors, i.e. bases are not measured. Therefore, α-HL nanopores with improved properties are desired.